1. Field of the Invention
This invention relates to a real pipelaying vessel. More particularly, the invention pertains to a new type of ship, specifically a self-propelled, dynamically-positionable reel pipelaying ship, in which a pipe spooling reel and associated pipe handling equipment are integrated into the ship's construction.
The vessel of this invention has been specifically designed to accommodate a permanently mounted pipe spooling reel substantially larger than any other pipe spooling reel heretofore known or used and capable of spooling substantially larger size pipe than any heretofore used.
2. History of the Prior Art
In laying offshore subsea pipelines for such uses as the gathering of oil and/or gas from offsea subsea wells, as, for example, in the Gulf of Mexico, it has been conventional to use one of two main methods to lay the pipe. In the first, or "stovepiping" method, a pipeline is fabricated on the deck of a lay barge by welding together individual lengths of pipe as the pipe is paid out from the barge. Each length of pipe is about 40' or 80' long. Thus, the pay-out operation must be interrupted periodically to permit new lengths of pipe to be welded to the string. The stovepiping method requires that skilled welders and their relatively bulky equipment accompany the pipelaying barge crew during the entire laying operation; all welding must be carried out on site and often under adverse weather conditions. Further, the stovepiping method is relatively slow, with experienced crews being able to lay onely one to two miles of pipe a day. This makes the entire operation subject to weather conditions with can cause substantial delays and make working conditions quite harsh.
The other principal conventional method is the reel pipelaying technique, in this method, a pipeline is wound on the hub of a reel mounted on the deck of a lay barge. Pipe is generally spooled onto the reel at a shore base. There, short lengths of pipe can be welded under protected and controlled conditions to form a continuous pipeline which is spooled onto the reel. The lay barge is then towed to an offshore pipelaying location and the pipeline spooled off the real between completion points. This method has a number of advantages over the stovepiping method, among them, speed (one to two miles per hour); lower operating costs (e.g., smaller welding crews and less welding equipment must be carried on the lay barge); and less weather dependency.
Historically, the technique of laying undersea fluid-carrying pipelines had its rudimentary beginnings in England in the 1940's. In the summer of 1944, 3" nominal bore steel tubes, electrically flash-welded together, were coiled around floating drums. One end of the pipe was fixed to a terminal point; as the floating drums were towed across the English Channel, the pipe was pulled off the drum. In this manner, pipeline connections were made between the fuel supply depots in England and distribution points on the European continent to support the allied invasion of Europe. (See Blair, J. S., "Operation Pluto: The Hamel Steel Pipelines", Transactions of the Institute of Welding, Feb. 1946.)
The broad concept of reel pipelaying was also disclosed in British Pat. No. 601,103 (Ellis), issued Apr. 28, 1948, wherein it was suggested that lengths of pipe be joined together at the manufacturing plant and coiled onto a drum, mounted on a barge or ship; the loaded barge would then be moved to the desired marine location and the pipe unwound from the drum by fixing one end of the pipe and towing the barge away from the fixed location.
While the concepts described in British Pat. No. 601,103 and those actually used in operation Pluto were adquate for wartime purposes, no known further development work or commercial use of the laying pipe offshore from reels was carried out after World War II. After a hiatus of about fifteen years, research into the reel pipelaying technique was renewed and was carried on by Gurtler, Hebert & Co., Inc., of New Orleans, Louisiana; by 1961, Gurtler, Hebert had sufficiently advanced the reel pipelaying technique to make it a commercially acceptable and viable method laying pipe in the offshore petroleum industry, able to compete with the traditional stovepiping technique. The first known commercial pipelaying reel barge, called the U-303, was built by Aquatic Contractors and Engineers, Inc., a subsidiary of Gurtler, Hebert, in 1961. The U-303 utilized a large vertical axis reel, permanently mounted on a barge and having horizontally oriented flanges (generally refered to in the trade as a "horizontal reel"). A combined straightener-level winder was employed for spooling pipe onto the real and for straightening pipe as it was unspooled. The U-303 first laid pipe commercially in Sept. 1961, in the Gulf of Mexico off the coast of Louisiana and was used successfully during the 1960's to lay several million linear feet of pipe of up to 6" diameter. The U-303 reel pipelaying barge is described in U.S. Pat. No. 3,237,438 (Tesson) and U.S. Pat. No. 3,372,461 (Tesson), both assigned to the assignee of the invention hereof.
The successor to the U-303, currently in use in the Gulf of Mexico and known in the trade as the "Chickasaw", also utilizes a large horizontal reel, permanently mounted to the barge such that it is not readily movable from one carrier vessel to another. Various aspects of "Chickasaw" are described in the following U.S. Patents, all assigned to the assignee of the invention hereof:
Sugasti, et al.--U.S. Pat. No. 3,630,461 PA1 Gibson et al.--U.S. Pat. No. 3,641,778 PA1 Mott, et al.--U.S. Pat. No. 3,680,432 PA1 Key, et al.--U.S. Pat. No. 3,712,100 PA1 1. A "turn" is that length of pipe wound through one complete revolution of the reel; PA1 2. A "wrap" comprises a plurality of turns making up a layer of pipe wound on the reel across the full or substantially full width of the reel.
Commercial reel pipelaying techniques require the use of certain pipe handling equipment in addition to the reel. Among such pipe handling equipment assential to any commercial reel pipelaying system is a straightener mechanism. This may take the form of a series of rollers or tracks, or any other arrangement which imparts sufficient reverse bending force to the pipe to remove residual curvature so that after unspooling, the pipe will lay substantially straight on the sea bottom. No such pipe-conditioning apparatus was used in Operation Pluto or contemplated by the British Ellis Patent.
U.S. Pat. No. 3,982,402 (Lang, et al.) describes an apparatus for laying pipe from a vertical reel in which the pipe conditioning apparatus is pivotable to adjust the lift-off angle of the pipe relative to the horizontal (e.g., the deck of a ship) as a function of the water depth in which the pipe is being laid. This has distinct commercial advantages, especially where the reel pipelaying system is incorporated into a self-propelled ship, such as that of the present invention, capable of traveling to different job sites, having different pipe size and/or lay depth requirements.
An early concept for a reel pipelaying ship is described in Goren, et al., "The Reel Pipelay Ship - A New Concept" Offshore Technology Conference Proceedings, May 1975 (Paper No. OTC 2400). This paper (hereafter the Goren, et al. 1975 OTC Paper) describes advantages and operating features of a proposed reel pipelaying ship. However, the cost of construction of a ship as described there was estimated to be on the order of $1,000,000,000; by contrast the ship of the herein described invention is currently under construction at less than one-third that estimate. The research and development work for the ship described in the Goren et al paper, (done at great expense by or on behalf of the assignee of this application) was subsequently materially revised in numerous major respects, and substantial changes and improvements were made to achieve the design of the substantially different reel pipelaying ship described hereinafter; this new reel ship is or will be materially different in concepts, construction features, mode of operation and results compared with the ship decribed in the Goren et al paper.